1. Field of the Invention
The present invention relates to a plastic optical element for use in optical scanning devices, which are used for image forming apparatus and optical apparatus. In addition, the present invention relates to an optical scanning device using the plastic optical element, and an image forming apparatus using the optical scanning device.
2. Discussion of the Related Art
Recently, in order to fulfill a need for producing high resolution full color images at a high speed, color image forming apparatus such as digital copiers and laser printers using a tandem optical device have been developed and commercialized. In such color image forming apparatus, plural (four) light beams irradiate plural (four) photoreceptors, which are arranged side by side in the receiving material feeding direction, at the same time to form electrostatic latent images on the photoreceptors. The electrostatic latent images on the photoreceptors are developed by the respective developing devices using different color developers (such as yellow, magenta, cyan and black color developers) to form visual color images (such as toner images) on the respective photoreceptors. The color images are transferred one by one on a receiving material optionally via an intermediate transfer medium to form a color image (such as full color images) on the receiving material.
Such image forming apparatus typically include plural optical scanning devices emitting respective light beams.
Such optical scanning devices typically use rectangular optical elements, which focus laser beams and perform various correction functions. Recently, in order to reduce costs of the optical elements, the material constituting the elements is changed from glass to plastics. In addition, in order to impart various functions to one optical element, the surface is changed from the spherical surface to non-spherical surface. For example, lenses, which have a large thickness and whose thickness is not even in the longitudinal direction thereof are typically used as optical elements.
An optical scanning device (and method) is proposed, in which plural light beams emitted by a light source are guided to one deflector to be scanned, and the scanned light beams are guided to respective focusing devices to focus the light beams on the respective photoreceptors. Each of the focusing devices includes a plastic optical element, and the focusing device, the light source, the incident optical system receiving the light beams emitted by the light source, and the deflector deflecting the light beams exited from the incident optical system are contained in a housing.
Molding methods such as gate seal molding methods and re-melt molding methods have been conventionally used for producing plastic optical elements. The purpose of using such molding methods is to prepare a plastic element with high dimensional precision while reducing internal strain to improve the precision of the resultant element. Specifically, after a resin is contained in a die, the resin is heated to a temperature not lower than the glass transition temperature of the resin and the temperature and pressure of the resin in the die are controlled to be constant. The resin is then gradually cooled to a temperature not higher than the thermal deformation temperature of the resin, and is released from the die, resulting in formation of a molded plastic element.
Although a plastic material with high dimensional precision can be prepared by using these molding methods, the molding time of the methods is much longer than that of injection molding methods in which the temperature of the die is maintained to be constant because it takes a long time to raise and drop the temperature of the die. Therefore, the molding methods have low productivity.
In contrast, a surface sink control molding method utilizing the high productivity of the injection molding method is disclosed in a Japanese patent No. 3,512,595 (i.e., published unexamined Japanese patent application No. (JP-A) 11-028745 corresponding to U.S. Pat. No. 6,015,514). In this method, a resin is contained in a die whose temperature is controlled to be not higher than the glass transition temperature of the resin while controlling a sink occurring in the molding process such that the sink does not affect the properties and dimensional precision of a functional surface (hereinafter sometimes referred to as transfer surface) of the resultant optical plastic element. In this method, in order to prevent occurrence of a sink on a transfer surface due to shrinkage of the molded resin, a surface (hereinafter sometimes referred to as imperfect transfer surface) of the molded resin other than the transfer surface is separated from a surface of the die so that shrinkage of the resin occurs at the imperfect transfer surface (i.e., shrinkage does not occur at the transfer surface) in the cooling process.
By using this surface sink control molding method, a plastic element with large thickness and/or uneven thickness can be molded at the same molding time as that of conventional injection molding methods without performing gradual cooling. In addition, the dimensional precision and internal strain of the resultant plastic element are as good as those of plastic elements molded by the above-mentioned molding methods performing gradual cooling. Further, since sinks can be certainly caused only on the imperfect transfer surfaces of the plastic element, the transfer surface of the plastic element has high dimensional precision. By using this surface sink control molding method, a plastic element having high molding stability can be prepared with little dependence on the pressure in the molding process.
The surface sink control molding method is effective for molding plastic elements such as fθ lenses which are thick in the light beam transmission direction. However, there are long plastic elements, which, unlike such fθ lenses, have a thickness (lens thickness (a) illustrated in FIG. 2B) in the light beam transmission direction smaller than the width (i.e., lens width (b) illustrated in FIG. 2B) in the sub-scanning direction. When the surface sink control molding method is used for molding such a long plastic element, a problem tends to occur. Specifically, since the cooling speed of the lens thickness direction is faster than the lens width direction in the cooling process, shrinkage of the long plastic element in the lens thickness direction becomes large, and thereby a sink is easily formed on the transfer surface of the long plastic element.
In attempting to solve the problem, JP-A 2007-133179 discloses a technique such that a projected portion is formed on another transfer surface of the plastic element, which is different from the transfer surface to be used as an optically functional surface, to increase the mold-releasing resistance of the element so that the sink does not enter into the reference surface of the plastic element, on the basis of which the plastic element is attached to a member.
Thus, when molding long plastic elements, it is necessary to sink an imperfect surface thereof more effectively than in the case of thick plastic elements (such as fθ lenses) so that the transfer surface (i.e., optically functional surface) thereof is formed with high precision.
Long plastic optical elements typically have a rib with thickness of few millimeters around the optically functional surface to increase the mechanical strength thereof, thereby preventing deformation of the elements even when receiving external forces. When an imperfect surface is formed on one side of the rib, the distance between the optically functional surface and the imperfect surface increases by the thickness of the rib. Therefore, a problem in that the effect of preventing formation of a sink on the optically functional surface by forming a sink on the imperfect surface is hardly produced occurs.
As mentioned above, by inserting a resin into a cavity of a die or by injecting a melted resin in to a cavity, plastic optical elements can be mass-produced at relatively low costs even when the optical elements have special shapes.
In the cooling process of such conventional molding methods, it is preferable to control the pressure and temperature of the resin in the cavity of the die so as to be even to produce a plastic element, which has a desired shape and high dimensional precision. In the case of a long plastic lens disclosed in JP-A 2007-133179, which has uneven thickness, volume shrinkage of portions of the lens is different depending on the thickness of the portions, resulting in deterioration of dimensional precision of the lens. In addition, a sink tends to be formed on a relatively thick portion of the lens.
In this regard, when increasing the pressure of the injected resin (i.e., by increasing the amount of the injected resin) in an injection molding method to solve this problem, the internal strain of the resultant plastic element increases. Particularly, when the plastic element is a thick optical element having uneven thickness, the internal strain seriously increases, thereby affecting the optical properties of the plastic element.
Namely, when the pressure of the injected resin is decreased (i.e., the amount of the injected resin is decreased) to decrease the internal strain of the molded plastic element, a problem in that a sink is formed on thick portions of the element occurs. In contrast, when the pressure of the injected resin is increased (i.e., the amount of the injected resin is increased) to prevent formation of a sink on the plastic element, a problem in that the plastic element has large internal strain occurs.
In attempting to solve the problems, JP-A 2000-329908 proposes a technique such that, as illustrated in FIGS. 3 and 4 thereof, a recessed portion (i.e., an imperfect transfer portion) is formed on a surface other that the transfer surface (i.e., optically functional surface) of the plastic element.
Particularly in a case of long plastic element in which a ratio (g/h) of the length (illustrated by a character (g) in FIG. 13B) of the element in the sub-scanning direction to the thickness (illustrated by a character (h) in FIG. 13A) of the lens portion is greater than 1, the cooling speed (i.e., thermal shrinkage) of the lens portion (i.e., a portion 301 in FIG. 13A) is faster than the other portions of the element, and thereby a sink is easily formed on the surface of the lens portion.
A long plastic lens is illustrated in FIG. 11.
Referring to FIG. 11, numeral 301 denotes the main body (i.e., lens portion) of the plastic lens, and numerals 302 and 303 respectively denote the entrance surface which is one of transfer surfaces and from which a light beam (incident light) enters, and the exit surface which is also one of transfer surfaces and from which the light beam exits. When the plastic lens is used for an optical scanning device, the plastic lens is set in such a manner that the longitudinal direction of the lens is identical to the main scanning direction of the optical scanning device as illustrated in FIG. 11.
FIG. 12 illustrates the entire of the plastic lens. Specifically, a rib 306 is formed on each of side surfaces 304 and 305 of the main body 301, which are different from the transfer surfaces 302 and 303. In addition, another rib (second rib) can be formed on each of the other side surfaces (i.e., the end surfaces in the main scanning direction), which are perpendicular to the side surfaces 304 and 305, although the second rib is not illustrated in FIG. 12. These ribs are molded while integrated with the main body 301 and made of the same material as that of the main body.
FIGS. 13A and 13B illustrate the plastic lens observed from directions S and R (illustrated in FIG. 12), respectively. In this regard, the direction R is the light entrance direction. In FIG. 13B, numeral 307 denotes the light transmission region of the lens, through which a light beam passes. FIG. 14 illustrates the cross section of the plastic lens when the lens is cut by a line C-C illustrated in FIG. 12 (i.e., when the lens is cut at any point thereof in the main scanning direction (x)).
JP-A 2000-329908 discloses a technique in that a recessed portion (i.e., an imperfect transfer portion) is formed on at least one of the ribs (e.g., ribs 306 in FIG. 14) to prevent occurrence of the sink problem even when the molded material is a long plastic lens having a ratio (g/h) of greater than 1 and the injection pressure is relatively low (i.e., the amount of the injected resin is relatively small).
However, even when this technique is used, the following problem tends to occur. Specifically, as mentioned below in detail, a problem in that since the length (g) of the lens in the sub-scanning direction is greater than the thickness (h) thereof, the surface 303 of the lens is slanted in the sub-scanning direction (as illustrated in FIG. 23), resulting in deterioration of precision of the lens occurs depending on the conditions of the formed recessed portion (i.e., imperfect transfer portion). In addition, when the slanting degree is different in the main scanning direction of the lens, the lens is twisted in the sub-scanning direction, resulting in distortion of the lens, thereby producing large variation of positions of light spots formed by scanning of light beams passing the lens.
Variation of positions of light spots, particularly, variation of positions of high frequency component, in an optical scanning device including such a lens and used for color image forming apparatus causes a color misalignment problem in that two or more color images are over laid while mis-aligned, resulting in formation of color images with poor color reproducibility. In this regard, the high frequency component means the residual component determined by subtracting the secondary component from the scanning position, and is an important factor in the qualities (such as color misalignment) of color images. This problem becomes remarkable recently because the recent image forming apparatus are desired to produce high resolution images.
Because of these reasons, a need exists for a molding method by which a long plastic optical element having an optically functional surface with high precision can be produced at relatively low costs even when the thickness of the element is smaller than the length thereof in the sub-scanning direction.